The invention pertains to the field of ion implantation equipment and, more specifically, to serial ion implantation equipment.
In ion implantation, a beam of energetic ions impinges upon a surface of material to imbed or implant those ions into the material. Ion implantation processes are categorized into batch and serial processes. Serial processes are the most common type of ion implantation processes, and are associated with medium dose implantation. Serial processes most often use a plasma ion beam that is subjected to electrostatic deflection processes in both axes normal to the direction of beam propagation. The electrostatic deflection processes are intended to provide a uniform distribution of ions in terms of density and direction of travel, but in practice ion beams vary in angle by as much as 3xc2x0 relative to the direction of beam propagation. This variance produces undesirable effects in the ion implantation processes, as reported in U.S. Pat. No. 4,726,689 to Pollock.
U.S. Pat. Nos. 5,406,088 and 5,229,615 to Brune et al. describe a parallel beam ion implantation device that was developed in response to increasing commercial use of large wafer diameters. The growth in wafer diameter from 4xe2x80x3 to 6xe2x80x3 and then to 8xe2x80x3 in diameter has generated a need for a serial implantation device capable of producing a beam that strikes the surface of the wafers with a uniform parallel beam while also permitting tilt and rotational control of the wafers.
U.S. Pat. No. 5,350,926 to White et al. describes a high current broad beam ion implanter with emphasis upon systems for beam control to establish uniformity across a large ribbon shaped beam traveling in a single transverse direction. The ion implanter uses a Freeman, Bernas, or microwave source, from which the ion beam is extracted from source plasma through a parallel-sided convex slot. The ion beam passes through a pair of analyzing magnets to render the beam parallel in both axes normal to the direction of beam propagation. U.S. Pat. No. 4,922,106 to Berrian et al. similarly shows an ion beam implantation device having a parallel beam generator together with mechanical and electrical scan controls that facilitate uniform implantation.
Hybrid scanning systems are the type most often used in modem serial processing ion implantation equipment. Processing occurs for one wafer at a time. As shown in FIG. 1, which is a midsectional side elevational view, it is common to mechanically scan a wafer 100 in one axis by passing the wafer 100 through a scanned ion beam 104, i.e., an ion beam 104 that is projected from source 102. The horizontal ion beam 104 has a transverse axis 106 with respect to the vertical axis 108 of wafer motion. The axis 106, as shown in FIG. 1 is an average representation of the beam axis. Portions of the ion beam 104 may be slightly off-axis due to beam shaping field elements, such as are shown in U.S. Pat. No. 5,350,926 to White et al. Generally, the wafer 100 is vertically translated along axis 108 through the horizontally scanned ion beam 104 as a means of distributing the ion beam uniformly over the wafer surface. It is necessary to setup the incoming ion beam 104 prior to implanting the wafer 100, in order to achieve uniform implantation by this scanning method. These processes occur in a beam implant vacuum chamber 110. A wafer holder 112 may comprise an arm, a linear conveyor, or any other type of wafer holder. The wafer holder 112 presents a wafer surface 114 that is available for ion implantation through the effects of ion beam 104.
As shown in FIG. 2, which is a midsectional top plan view, setup of the scanned ion beam 104 for uniform implantation is accomplished by sampling with a faraday cup 200 that moves horizontally across the full beam width W in a direction that is normal to the beam axis 106 at the setup plane 202. The setup plane 202 is ideally located where the wafer implant occurs on surface 114 (see FIG. 1). The faraday cup 200 is deployed at a plurality of sampling stations, e.g., stations 204 and 206, to provide a fair representation of the beam uniformity at all positions on setup plane 202. Ion beam current collected by the faraday cup 200 is measured as a function of faraday cup position. Subsequent adjustments to the ion beam optical elements in source 102 are made by conventional means to even out the beam current, e.g., as taught in U.S. Pat. No. 5,350,926 to White et al. Measurement of beam current and adjustment of the ion optics are repeated according to conventional practices until the beam current is uniform within acceptable limits.
As shown in FIG. 3, hybrid implantation systems have process requirements that mandate control of the angle 300 of ion beam incidence with respect to the wafer surface 114 during implantation, for example, as described in U.S. Pat. No. 5,898,179 to Smick et al. This control is usually accomplished by tilting the wafer 100 within the wafer holder 112. Tilting occurs with respect to the trajectory of ion beam 104 and the mechanical scan axis 108. This tilting produces an angle 300 of incidence between the incoming ion beam 104 and the wafer surface 114 that is constant everywhere on the wafer. The mechanical translation of wafer 100 continues, as before, in a vertical direction along axis 108. The incident angle 300 generally ranges from 0xc2x0 to 45xc2x0 and is measured in the y-axis plane between the ion beam trajectory along axis 106 and the axis 304 that is normal to the implanted wafer surface 114. For example, a 0xc2x0 implant angle occurs when the wafer implant surface 114 is oriented at 90xc2x0 relative to the ion beam trajectory along axis 106.
Tilting the wafer 100 with respect to the mechanical scan axis 108 can have a deleterious effect on the uniformity of ion implantation because some regions of the wafer surface 114 are not implanted in the same focal plane as the setup plane 202. These problems are exacerbated by the current trend of using larger wafers, so that distances between the setup plane 202 and the plane of surface 114 can be significant. Where the wafer 100 is tilted by rotation relative to the mechanical scan axis 108, one end 306 of the wafer rotates toward the incoming ion beam 104 while the other end 308 rotates away. The middle region 310 of the wafer 100 remains in the setup plane. If, for example, the horizontal tilt axis is located entirely below the wafer 100, then the entire wafer moves out of the setup plane 202. Ion beam current uniformity is not specifically known other than in the setup plane 202 where it was actually measured. Therefore, the implant and setup planes should be coplanar.
The ion beam 104 contains positively charged plasma particles, which impinge upon surface 114 to impart a net charge on wafer 100. The effects of this imparted charge are cancelled, according to conventional practices, by utilizing a flood gun 312 to emit an electron stream 314. An exemplary ion implantation system including a flood gun for use in neutralizing accumulated plasma charges is the VIISta 80 ion implanter that is produced by Varian Semiconductor Equipment of Glouchester, Mass., as described, for example, in Radonov et al., In Situ Charging Potential Monitoring for a High Current Ribbon Beam (a Varian Trade Publication 2001). The electron stream 314 impinges upon wafer 100 to cancel the net charge. As wafer 100 is tilted in increasing magnitude of angle 300, surface 114 is increasingly exposed to the electron stream 314, and there is a corresponding increase in contact from electron stream 314 with associated net charge effects on wafer 100. Similarly, surface 114 is less exposed to the ion beam 104 by virtue of this tilting with associated net charge effects on wafer 104. These net charge effects, in combination, produce problematic localized field distortions that vary the uniformity of ion beam 104 as a function of the magnitude of angle 300 and related variances in the travel distance for ion beam 104.
Attempted improvements to tilt-scanning systems include adjustments to the wafer moving systems such that the wafer translational axis moves as a function of angle 300 to a new axis 108xe2x80x2. The entire wafer scanning apparatus in chamber 110 is tilted on a horizontal axis to accomplish this effect. This tilted displacement of the translational axis 108 to axis 108xe2x80x2 assures that the center of ion beam 104 impinges upon all points of surface 114 at a constant focal distance regardless of the magnitude of angle 300. Accordingly, the method produces parallel scan implants at a selected angle 300 without implanting outside of the beam focal plane. Setup of ion beam 104, according to these improved methods, proceeds horizontally as shown in FIG. 2.
These improvements are associated with numerous problems, such as an increased length of scan stroke along axis 108xe2x80x2, which results in significantly increased mass, complexity and cost in constructing the motive system within vacuum chamber 110. Wafer handling structures, such as wafer feeding and disposition systems, must be able to work in cooperation with the tilted axis 108xe2x80x2 at a variety of angles. The flood gun 312 is usually positioned so close to wafer 100 that the flood gun interferes with the motion of wafer handling and transfer systems in the implantation chamber 110. System reliability and repeatability are reduced by these complexities, and wafer handling capacity is reduced.
The hybrid scan system and method of the invention solves the problems outlined above by providing an ion implantation system that achieves uniformity when tilting wafers out of the setup plane without tilting the entire mechanical scan axis. In summary, the system deliberately tilts the wafer out of the typical setup Faraday sample plane to provide an implant angle. A two-axis faraday performs the beam setup in the implant plane at the exact implant angle, to eliminate out-of-focal plane problems. The overall system is advantageously simpler, smaller, more reliable and less costly to use than are prior systems.
The ion implantation system includes a source of ions that are scanned linearly along a first axis to produce an ion beam, such as a parallel path fan beam having a two dimensional cross section that is normal to the first axis and at least twice as large in one dimension than another. A workpiece holder, such as a wafer holder, is configured for mechanical scanning in linear motion along a path of motion perpendicular to the first axis. This configuration is achieved, for example, through the use of a first vertically extensible drive arm that is rotatable about its axis of extension. Selectively adjustable rotation control structure is utilized for rotating the workpiece using the direction of the vertically extensible path of motion as an axis of rotation to orient an implant surface on a workpiece at a selected angle of rotation when the workpiece is installed in the workpiece holder. A beam measuring device, such as a Faraday cup, is configured for scanning along an intended location of the implant surface to provide a setup measurement coincident with the intended location. Thus, the setup plane of the beam measurements is not perpendicular to the direction of ion beam propagation when the workpiece holder is rotated.
Other aspects and instrumentalities include using at least one charge neutralization device, such as an electron flood gun or a plasma bridge, which is directed towards the workpiece holder for neutralization of beam charge buildup. A rotatable mechanism is configured to maintain the charge neutralization device in corresponding rotational alignment with the workpiece holder. For example, the charge neutralization device may be mounted on a second vertically extensible drive arm that is rotatable about its axis of extension. The rotatable mechanism associated with the charge neutralization device and aligned with the first vertically extensible drive arm such that the rotatable mechanism can be rotated in linear alignment with the selectively adjustable rotation control structure. This alignment maintains an orientation of the angular rotation and spacing of the charger neutralization device and the workpiece holder.
The foregoing system is used in a method for ion implantation of a workpiece comprising the steps of generating an ion beam perpendicular to a first XY plane having an X-axis and a Y-axis; scanning the beam across the workpiece along the X axis of the first XY plane; identifying a second plane by rotating the first XY plane about the Y-axis; measuring the effective ion beam intensity along a line in a second plane to provide a beam intensity signal; and adjusting the ion beam based upon the beam intensity signal to obtain an adjusted ion beam having a more uniform ion beam intensity along the line in the second plane; rotating the workpiece to present an implant surface in alignment with the second; and translating the workpiece along the Y axis of concomitantly with the XY planes to pass the workpiece through the adjusted ion beam to accomplish ion implantation in the workpiece. The method may further comprise the steps of positioning a charge neutralization device in a position of rotational alignment with the workpiece prior to the step of rotating the workpiece; and re-aligning the charge neutralization device into the position of rotational alignment after the step of rotating the workpiece.
The foregoing system and method offer several advantages. The implant plane and the setup planes are coplanar and unaffected by beam height and/or implant angle. The scan axis is not tilted at all so scan stroke is minimized and wafer exchange height is typical as well as easily optimized. Because tilting is achieved by rotating a much smaller inertial mass it can be done quickly to maximize wafer throughput. Because the tilt motion is not used to move wafers between the implant and wafer load positions, the required range of tilt axis motion is driven only by implant angle requirements; 45 degrees instead of 90. This reduces the time required to exchange wafers and hence increases wafer throughput. The smaller inertial mass to be tilted also means that the motion can be produced with less powerful drives which are smaller and less costly. The optional flood gun is easily positioned close to the wafer flood gun-to-wafer geometry kept constant over the full range of implant angles. This is accomplished by simply mounting the flood gun from the ceiling of the implant chamber and rotating it about the scan axis to match the implant angle.